1. Field of the Invention
The present invention relates to hearing aids. The invention further relates to a method for communication between two parts of a hearing aid. The invention more specifically concerns a two part hearing aid comprising power supply means and at least one microphone for transforming an acoustic signal in the surroundings of a hearing aid user into an electric signal.
The hearing aid comprises a base part to be arranged outside the ear canal of a hearing aid user, said base part comprising signal processing means. The hearing aid also comprises an ear plug part to be arranged in the ear canal of a hearing aid user, said ear plug part comprising acoustic output means for transmitting sound into the ear canal, an ear canal microphone for transforming an acoustic signal in the ear canal into an electric signal, and an electronic module connected to said ear canal microphone. The hearing aid further comprises an elongated member connecting said ear plug part with said base part. The elongated member comprises electrical wires adapted for providing power supply from the base part to the ear plug part, or, from the ear plug part to the base part.
Hearing aids are often made as a two part device with one part, an ear plug, for being arranged in the ear canal of the hearing aid user, and another part, a base part, for being arranged outside the ear canal. Often the base part is arranged behind the ear, known as a behind-the-ear hearing aid. The base part will usually comprise signal processing means, one or two microphones and a battery. Often a receiver is also arranged in the base part. A sound tube will then connect the base part with the ear plug part, and sounds from the receiver will be transmitted through this sound tube to the ear plug part transmitting the sound further to the ear drum of the hearing aid user.
2. The Prior Art
In a known alternative the receiver is arranged in the ear plug part and connected with the signal processing means in the base part through e.g. two wires. In this case the sound tube is replaced by electric leads, suitably encapsulated.
It has been suggested to arrange a microphone in the ear plug, at the side proximally to the tympanic membrane, for transforming sounds in the ear canal into electrical signals. Such a microphone may have many purposes during fitting and during daily use of the hearing aid. The electrical signal from such a microphone needs to be transferred to the signal processing means of the base part of the hearing aid, normally by an extra pair of wires. It has now been realized that one problem in having such a microphone is that the wires used for transferring the signal from the microphone to the base part will gather electrical noise. The electrical signal generated in the microphone is relatively weak, e.g. 5-10 μV, and therefore rather sensitive to noise.
It has also now been realized that this problem is larger when a receiver is arranged in the ear plug, since the wires supplying the receiver signal, which may be 2 V at peak level, will be arranged close to the wires transferring the signal from the microphone. Therefore, it is likely that the receiver signal will induce noise into the wires carrying the microphone signal.
Another problem is that the number of wires preferably should be as low as possible in order to keep the diameter of the elongated member connecting the two parts as small as possible.